"half inches deep, drilled into the sills at intervals of eighteen inches asunder. This part of the railway "has recently been carefully examined by the engineer.

The severe winter of 1831-2 (during which the frost "was intense, and several thaws occurred) has not, "in the slightest degree, affected the stability of the "rails."

CHAPTER III.

ON THE STRENGTH AND STIFFNESS, AND BEST FORM OF SECTION, OF CAST AND MALLEABLE IRON RAILS.

§ 1.-General Remarks on Material best adapted for Railroads.

SINCE cast iron superseded the use of wood for rails, it has been most extensively used, in the construction of railroads. As usual in like cases, at its first introduction, considerable opposition was made to its use, its brittleness and liability to break, its cutting the wheels, when in the form of edge-rails, and several other objections, were urged against it; time and experience have, however, confirmed its utility, and extirpated those prejudices. Though its nature renders it liable to break, when subjected to sudden blows, and its strength is considerably affected, by the unavoidable occurrence of air bubbles, and other imperfections in its organisation; yet still we are enabled to form a railroad with it, on which weights of considerable magnitude, can be conveyed, at moderate rates of speed, without much risk of breakage. And as it is a consideration of paramount importance, in the construction of a railroad, to form it of such materials, as combine strength and durability with economy; cast-iron is superior to timber.

Cast iron, while its hardness presents a surface that opposes little obstruction to the wheels of the carriages, forms a substance, which is also very durable, and resists the action of the wheels, with great effect. Its brittleness forms the only source of reasonable objection; and,

as this cannot be obviated, without increasing the section of the rail, and adding to the weight, and consequently to the cost, it has led to the substitution of malleable iron rails, the tenacity of which resisting sudden fracture, obviates the danger, inconvenience, and cost of the breakage of cast iron.

In describing the different kinds of rails, used in the construction of railroads, we have previously given the opinion of some engineers, on the comparative merits of cast and malleable iron rails, offered previously to 1825. Since that period wrought-iron rails have been more extensively, or almost exclusively, used upon all the public lines of railways; and the prejudice, which then existed against their adoption, experience has since dispelled. There were then wanting experiments, on the comparative strength, durability, and the resistance to the carriages moved along them; which the numerous applications of this mode of transit, have since presented many opportunities of furnishing.

We shall now endeavour to supply these deficiencies, and in doing so, we shall first of all endeavour to determine the strongest form of section, which applies equally to both cast and malleable iron rails. This question, however, involves several considerations, we require, first of all, a certain breadth of bearing surface, for the wheels to run upon, and that breadth must be such as not to produce unnecessary wear in the wheels, nor yet too great, so as to make them unnecessarily heavy; then there must be a certain thickness, or depth of that bearing surface, to make the rail sufficiently durable; next, the depth of section must be such, as to render the rail sufficiently rigid. All these requisites

must, therefore, be considered, in determining upon the proper form of section.

Two, or two and a half inches in breadth, at the top, seems to be established, as the proper width for the wheels, to run upon; the latter being the breadth, adopted on all the public railways in Great Britain. The strength of rails, being as the breadth, and square of the depth, a greater breadth, than what is absolutely necessary, is, therefore, adding to the weight of the rail, without increasing the strength, more than in the direct ratio of the breadth; whereas, the same quantity of material, disposed in terms of the depth, increases the strength, in the duplicate ratic.

It does not appear, from the experience of the wear of the rails, and wheels, that they should be of less breadth than two inches; nor does it appear necessary, to make them of greater width, than two and a half inches.

We shall now, give some of the results of the wear, of cast and wrought iron rails, with a view of determining the depth of bearing surface.

In the former edition of this Work, we gave an account of two experiments, on the wear of cast and wrought iron rails, upon the Stockton and Darlington railway, as follows:-Malleable-iron rails, fifteen feet long, over which locomotive engines pass, weighing from eight to eleven tons; waggons loaded, four tons each; 85,000 tons passed over in a year, exclusive of engines and empty waggons; weight of rail, 136 pounds; loss of weight, in twelve months, eight ounces; the breadth of the top of the rail, being two and a quarter inches, gives one tenth of a pound, per yard per annum; and Mr. Story informs us, that subsequent experiments furnish nearly the same result. In determining the premium for the best form of rail, for the

London and Birmingham Railway Company, with Professor Barlow, and Mr. Rastrick; we found the annual wear, estimated by some of the competitors, at one sixth of a pound, per yard, per annum. Upon the Killingworth railway, I have had some of the rails, which were weighed, and laid down, in 1825, taken up, and re-weighed; and find the average loss of weight of several rails, to have been eight pounds, for each fifteen-feet rail, in twelve years, which gives about one eighth of a pound, per yard, per annum. These rails were laid down, at a time, when the manufacture of malleable-iron rails, was not so well understood as at present; and, on examining, I found part of the loss of weight was attributable, to exfoliation on the sides. About 100,000 tons of coals would pass over these rails, annually, exclusive of the weight of the engines and empty carriages. Mr. Dixon, the resident engineer upon the Liverpool and Manchester railway, states, the wear of the rails, upon that railway, to be one tenth of a pound, per yard, per annum, which was determined, by taking up three rails, cleaning and weighing them; and then, at the end of twelve months, taking them up again, cleaning and weighing them, as before; and this being repeated, for two years, the wear was found to be the same. [Note A. Appendix.]

We may, therefore, take the wear of the rails to be about one tenth of a pound, per yard, per annum, which, supposing the whole to result from the wear, on the upper surface, will be one eighty-fourth part of an inch; if the top, or wearing part, of the rail were, therefore, an inch in depth, the rail would wear eighty-four years. The whole of the wear, above alluded to, does not, however, take place upon the top; a part, though, probably, a very small portion, is attributable to exfoliation, by the action of the air: